1. Field of the Invention
This invention relates to photographic objective lenses and in particular to such lenses that comprise three air spaced elements.
2. Description of the Prior Art
Triplet lenses for use in photographic apparatus are well known and have been used for many years. Moreover, high relative aperture lenses have also been in use for many years in the photographic industry. However, with the advent of available light photography, the use of high relative aperture lenses has spread to photographic apparatus designed to be mass produced for the amateur market. The price of such lenses then becomes of great concern and the relatively complex, well corrected, high relative aperture lenses known in the art are too costly.
The introduction of plastic elements in triplet lensens has been generally adopted in order to decrease the cost of such lenses. However, the introduction of plastic elements has usually resulted in accentuation of certain design problems. For instance, variation in the back focus length due to thermally induced contraction and expansion of the optics may be increased.